Superabsorbents are water-insoluble crosslinked polymers which are capable of swelling and forming hydrogels to absorb large amounts of aqueous liquids, especially body fluids, preferably urine or blood, and of retaining them under a particular pressure. By virtue of these characteristic properties, these polymers are employed principally in incorporation into sanitary articles, for example nappies, incontinence products or sanitary towels.
Superabsorbents are generally produced by the free-radical polymerization of monomers bearing acid groups in the presence of crosslinkers. Through the selection of the monomer composition, of the crosslinkers and of the polymerization conditions and the processing properties for the hydrogel obtained after the polymerization, it is possible to produce polymers with different absorber properties. Further possibilities are offered by the production of graft polymers, for example using chemically modified starch, cellulose and polyvinyl alcohol according to DE-A 26 12 846.
EP 876 888 A1 discloses a process in which the hydrogel is frozen and comminuted by means of a system consisting of rotating blades, and then dried and ground. The advantage is said to be that the pore structure is preserved since it is frozen before the first coarse comminution. A disadvantage here is that it is in principle necessary here to use a foaming agent in order to obtain the surface pore structure. In addition, a higher proportion of ultrafine particles is obtained in this comminution of the frozen hydrogel. Likewise, a higher energy expenditure and also a more complex technical solution are required for the comminution of the cooled hydrogel.
For formation of what are called “absorbent” polymers, polymerization of different kinds of normally water-soluble monomers is required, often also together with water-insoluble comonomers in the presence of crosslinkers. The crosslinkers are added during or after the polymerization. Such absorbent polymers are lightly crosslinked, water-insoluble hydrogel polymers which, in the dry and essentially anhydrous state, have a great ability to absorb water. This can amount to several times their own weight. Due to the high absorption capacity, the absorbent polymers are suitable for incorporation into water-absorbing structures and articles, for example nappies, incontinence products or sanitary towels. These absorbent polymers are also referred to in the literature as “superabsorents”. In this context, reference is made to Modern Superabsorbent Polymer Technology; F. L. Buchholz, A. T. Graham, Wiley-VCH, 1998.
The demand for thinner hygiene articles has increased markedly in the last few years. Predominantly thinner nappies which contain less fluff pulp are now being produced. The task of the fluff pulp accordingly has to be assumed by the absorbent polymer without any resulting loss of quality. Such polymers are produced in the presence of solvents, preferably water.
In order to optimize the properties, especially the absorption properties, of the superabsorbents for use in hygiene articles, the polymer particles, polymer gel obtained after drying and after comminution, are modified, preferably surface modified. The surface modification can give rise to a core-shell-like morphology, which is preferred especially in the case of superabsorbent particles. This modification serves, for example, to provide the superabsorbents with odour-binding properties, to improve the dusting characteristics of the superabsorbents, to reduce the caking of the superabsorbent particles, to improve the absorption capacity of the superabsorbents under pressure stress and/or to advantageously affect the permeability properties of the superabsorbent.